Author Topic: Sizing question  (Read 4118 times)

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Offline lar49

Sizing question
« on: January 23, 2011, 01:44:14 pm »
This may seem silly but how are touring bikes sized.  My Trek 5500 Road bike is 56cm from the center of the crank to the top of the seat post.  Is this the size I should be looking for in a touring bike or are they sized differently?

Offline John Nelson

Re: Sizing question
« Reply #1 on: January 23, 2011, 03:28:10 pm »
Sizing is done somewhat differently by different manufacturers. Touring bikes, however, are generally sized the same as any other bike. Generally the size is the measurement from the center of the crank to the center of the top tube as measured along the seat tube (C-C). If the bike has a sloping top tube, then the measurement is to the center of the virtual top tube, i.e., where the top tube would hit the seat post if the top tube was horizontal. Some manufacturers measure to the top of the top tube rather than the center of the top tube (C-T).

In the old days, the measurement was to the top of the seat tube.

See the following for more information:

http://www.sheldonbrown.com/frame-sizing.html
http://www.coloradocyclist.com/bikefit

The only way to know, however, if a bike fits you is to (1) ride it (a lot), and (2) have the fit checked by a good bike fitter. Work with a good bike shop that is committed to make it fit well (and not just get your money and get you out the door).
« Last Edit: January 23, 2011, 03:33:48 pm by John Nelson »

Offline rvklassen

Re: Sizing question
« Reply #2 on: January 24, 2011, 12:43:46 pm »
While nominal size is (as described above) crank centre to (virtual) top tube centre, the more important measurement is the top tube length, as it is unusual for the top tube to be too high if the top tube is short enough.  To know that you need to check the frame geometry, which is sometimes posted, and sometimes not.  Adjusting for top tube length is somewhat possible through swapping stems, but this gives 1cm increments over about a 10-15 cm range; adjusting for height is possible over a much larger range, and arbitrarily fine increments.

Offline MIBIKER

Re: Sizing question
« Reply #3 on: January 25, 2011, 10:31:41 am »
I would find out what the stand over height of the top tube. Compare this to your inseam or your current bike. It is best to have no less than one inch.  To find this measurement locate the middle of the top tube. (the half way point between seat and headset).  Then measure from this point to the floor this is the stand over height.

Offline DaveB

Re: Sizing question
« Reply #4 on: January 26, 2011, 06:09:39 pm »
While some level top tube bikes are size specified by the distance from the center of the bottom bracket spindl to the center of the top tube (c-c) many are specified by the distance from the center of the bottom bracket to the top of the top tube (c-t) and the difference is about 1 cm.  So a 54 c-c frame would be about a 55-c-t.   Sloping top tube bikes are harder to size since you need the "equivalent" based on a theoretical level top tube. 

Stand-over height is a useful measure since you need adequate clearance for comfort and safety but it isn't the only consideration.  Top tube length is a major consideration since it defines your reach to the bars and stem length can only compensate so far.  You don't want to require a very short or vary long stem because the top tube length is wrong for you.


Offline Pat Lamb

Re: Sizing question
« Reply #5 on: January 26, 2011, 06:59:34 pm »
Are you going to be working with a bike shop, or buying through the web?

If working with a bike shop, I'd pull them in early to help size you.  If you've got a good LBS, I highly recommend this route; not only can they put you on a decent sized bike, but they should take the hit if it doesn't fit; they can put everything together; and they can check everything over to make sure your bike is in good mechanical condition before you leave.  A really good bike shop may be worth stretching "local" to a few hours' drive, if you can try the bikes and find one that fits you.  Then all the previous information in this thread is moot.  ;)

If they're not that good (i.e., full of racer-boys or mountain bikers who can't or won't spell "touring"), or money is really tight, you may have to figure out sizing yourself.  Use the sizing links given above, but start by measuring yourself (maybe with a helper) and your current bike.  Sizing is generally similar between tourers and other road bikes, with two exceptions.  First, as noted above, you may have to translate between C-T, C-C, or some virtual measurement (good luck, precise measurement to an imaginary point in space is rather difficult to replicate).  Second, you may want to go for a size larger to get more stack space on the frame, which lets you get the bars higher.  IIRC, the 5500 is built with low bars; many (although not all) tourists prefer to have the bars up a bit and sit upright a bit more.

Note you'll need to be or hire a good mechanic to put everything together if you choose the second option.